A side airbag device which includes an inflator for injecting gas into an airbag, a plastic cover covering the airbag and the inflator at the front side, and a metal box for mounting the inflator to the cover is hitherto known in the art (see JP 2009-40328 A). To be more specific, in this technique, when the airbag is inflated by the inflator, the airbag will break a thin-walled portion of the plastic cover to thereby inflate and deploy.